In the imaging, lighting, display and electronics industries, it is predicted that in order to meet consumer demands, and fuelled by industry competitiveness, electronics products will be required to be increasingly durable, thin, lightweight and of low cost. In a growing market where consumers are demanding more from portable electronic devices and displays such as mobile phones, laptop computers, etc., flexible displays and electronics have the potential to eliminate the rigid constraints of traditional flat panel displays and electronics products. The goal in displays and electronics is to produce thin, lightweight, flexible devices and displays with achievable power requirements at a minimal cost.
Traditionally electronic devices requiring multiple layers of circuits have been fabricated using multiple circuit boards, with circuitry formed on one or both sides thereof, which may be bonded together and connected to one another by drilling holes (or vias) in the circuit boards which are filled with conductive material. To make such multiple layer circuit boards, a copper coated insulating board made of a composite material is treated with a light-sensitive material, known as a photoresist, which is imaged with the pattern of the desired electronic circuit, typically by exposing the photoresist through a photomask. The resist is affected by the exposure such that the exposed and non-exposed parts can be differentiated in terms of ease or method of removal. The imaged resist is then treated to remove the resist in an image-wise manner to reveal bare copper. The bared copper is then etched away and then the remaining resist removed to reveal a copper track on the insulating board. A second board may be made in a similar way with its own circuit pattern and the two boards bonded together and optionally connected by drilling vias as mentioned above.
The process of making electronic circuit boards such as this can be quite laborious and involves several sequential steps. A problem with making multiple layer circuit boards in this way, especially boards of greater than one meter across, is that it is not always possible to register the boards accurately. The photomasks used to image the photoresist-coated copper-clad boards are often subject to dimensional instability. For example, a mask, which is usually a photographic film, typically exhibits a humidity expansion coefficient of about 0.0012% per percent relative humidity, such that on a 5% change in relative humidity, a one meter photomask will expand or contract by about 60 μm. Polyester film has a thermal expansion coefficient of 0.0018% per ° C., such that a 5° C. change will result in a dimensional change of 90 μm and the effect of humidity expansion and thermal expansion can be cumulative. The conductive track resolution available by this method is therefore severely limited by the dimensional instability of the photomask, in that if it was desired to connect tracks of 50 μm width on one circuit board with that of another circuit board by drilling a via, a possible positional error of greater than 50 μm may hinder connection.
It is desirable to provide a solution to overcome the problem of registration in printed circuit board manufacture, to improve the efficiency of the electronic circuit manufacturing process and to enable electronic circuits to be generated on flexible supports to meet the predicted growth in demand for flexible circuits and flexible and thin devices. A number of attempts to provide new manners of manufacturing electronic circuits have been previously disclosed.
U.S. Pat. No. 4,469,777 relates to a process for preparing a two-layer printed circuit having conductive interconnections, via a singe exposure. According to the process described, at least one layer of a photoadhesive material is laminated onto a substrate bearing an electrically conductive circuit and then exposed to actinic radiation through a photomask of three different optical densities, one that transmits substantially all the radiation, one that transmits substantially no radiation and one that transmits an intermediate amount of radiation. The areas of the coated substrate exposed to substantially no radiation are removed by application of a suitable solvent in which the exposed photoadhesive material is insoluble, to form holes for vias. Finely divided metal, alloy or plating catalyst is applied to the adherent image areas (optionally tackified by heating) which correspond to the areas exposed to light of intermediate optical density and which correspond to a desired circuit patter and to the holes to form interconnecting vias (connecting the circuit pattern on the substrate to the circuit pattern on the surface of the laminated photoadhesive material). The pattern is then plated to generate an electrically conductive circuit pattern interconnected with the underlying pattern. Further layers of circuit may be formed by repeating the process.
U.S. Pat. No. 5,384,230 describes a method of fabricating printed circuit boards whereby the surface of a circuit board is covered with a photoresist layer and the photoresist layer in turn covered with a silver halide emulsion layer. The silver halide emulsion layer is then exposed according to a desired circuit board pattern with white light and the image developed to form a high definition mask in direct contact with the resist. The board is then exposed to UV light through the imaged emulsion layer, which is then stripped and the exposed photoresist-coated board processed in the conventional manner.
U.S. Pat. No. 2,854,386 relates to a method of photographically printing conductive metal patterns. As described therein, a thin layer of a photographic silver halide emulsion coated onto a support is exposed according to a desired pattern through a master transparency to generate a latent image which forms a dense and visible silver image upon development, preferably with a high contrast, non-fogging developer. The visible silver image formed is a negative of the final desired pattern. An oxidising etch solution is then applied which oxidises the metallic silver and simultaneously softens the associated gelatin thereby removing the gelatin from the support to leave a residual gelatin image. A latent silver image is formed in the residual gelatin image by re-exposing the whole support to actinic radiation and the silver nuclei act as seeds in the subsequent physical development step to form a heavy continuous conductive silver deposit. The resulting conductive silver pattern may be plated with copper or other metal according to standard electroplating techniques. In a second described embodiment, the emulsion is of a wash-off type emulsion comprising unhardened gelatin and a light sensitive tanning agent. The emulsion layer is exposed according to the desired pattern and non-pattern areas of gelatin removed by a wash-off developer to form a gelatin image of the desired pattern. A preliminary silver image is formed in the gelatin image by treating it with an alkaline solution and a silver salt, such as silver nitrate, whereby silver oxide particles are formed in the gelatin image, which then form the nuclei for forming a silver deposit via a physical development process and may optionally be electroplated with copper or other metal.
U.S. Pat. No. 2,195,531 describes a coating of nitrocellulose containing a photosensitive compound (silver bromide) and a conductor (carbon) on a cellulose acetate support, which is exposed according to a desired pattern and then developed, fixed and washed. The conductivity of the film depends upon the amount of carbon and the amount of reduced metal compound therein. The developed element may be used as a resistance element or variable resistor.
U.S. Pat. No. 3,223,525 describes a method of manufacturing, by photographic means, external electrically conductive noble-metal patterns on non-conductive supports. In the described method, a non-conductive support is treated with a light-sensitive compound such as silver halide, exposed to light to produce a silver or mercury germ image, which is then treated with a stabilised physical developer for a prolonged period of time whereby the internal image is made to grow out beyond the surface of the support to become an external image having resistivity of less than 104 ohms per square.
U.S. Pat. No. 3,929,483 describes a method by which one-sided and/or two-sided plated through-conductive circuit boards useful for printed circuits may be produced. An anodized aluminium sheet sensitised (on one or both sides) with silver salts is exposed according to a circuit pattern and developed, optionally with a physical developer, to generate a silver image. This is treated with hypochlorite solution and the resulting pattern plated with a metal to form conductive tracks. Where tracks are formed on both sides of the support, they may be connected by drilling through the support using standard techniques or by utilising a pre-drilled support.
DE 198373 relates to the production of conductive strips, resistors, and capacitors by photographic means. It describes a photosensitive material comprising an insulating layer support coated with two photosensitive, fine grain silver chlorobromide emulsion layers having a gelatin/silver ratio of 1:3 and being sensitive to the blue, green or red region of the spectrum (optionally to different regions). The two silver chlorobromide emulsion layers are separated by a dielectric layer, which is permeable to photographic development baths and which preferably has a thickness of between 3 and 4 μm and optionally contains dielectrically active metal oxides such as TiO2, Al2O3 and SiO2. Exposure of the emulsion layers through a mask according to the desired conductive pattern in each layers followed by development using a photographic developer with a development accelerator leads to formation of respective electrically conductive layers. Connectivity between the upper and lower electrically conductive layers formed from the respective emulsion layers can be effected by applying the intermediate dielectric layer in strips, with the required intermediate spaces.
U.S. Pat. No. 3,647,456 relates to a method of making electrically conductive silver images with the object of providing such electrically conductive silver images having high spatial resolution, which conducting silver image may be advantageously utilised in printed circuit techniques thereby eliminating the need for an aluminium layer in photoresists and establishing a silver pattern directly upon a wafer. There is described the use of a coating of silver bromide emulsion comprising cadmium iodide on a substrate to produce a latent image on the substrate, developing the latent image using a high resolution developer to provide a silver image and heating the silver image at a temperature of from 200° C. to 450° C. to render the silver image electrically conductive.
U.S. Pat. No. 6,517,931 describes a method of using a conductive silver ink in the manufacture of multi-layer ceramic capacitor (MLC) devices. The silver ink described typically comprises at least a high purity silver powder having an average particle size of up to 1 μm; an inhibitor such as a barium titanate based material; and a vehicle comprising a mixture of resin (e.g. ethyl cellulose) and solvent (e.g. toluene/ethanol mixture). According to U.S. Pat. No. 6,517,931, the ink is screen printed to a desired pattern on dielectric green tapes which are stacked to form a registry, laminated under pressure and then fired to form the MLC device.
The various alternative methods of generating printed circuit patterns illustrated in the above-referenced documents each has advantages as described therein, but they do not provide a solution to overcome the problem of registration in printed circuit board manufacture, to improve the efficiency of the electronic circuit manufacturing process, to enable narrow tracks and/or narrow gaps of less than 25 μm and to enable electronic circuits to be generated on flexible supports to meet the predicted growth in demand for flexible circuits and flexible and thin devices.